This invention relates to an improved method of anodizing aluminum and its alloys without the use of chromium-containing chemicals. More particularly, the invention relates to a method of using aqueous solutions of sulfuric and boric acids to achieve desired coating weights under well controlled conditions. Aluminum alloys are susceptible to corrosion, especially in a saline environment. Currently, the preferred method of protecting aluminum and its alloys from corrosion is to form a layer of aluminum oxide about 1 to 3 microns (about 200 to 600 mg/ft.sup.2) thick by anodizing in a chromic acid solution. This oxide coating is then sealed in hot deionized water or dilute chromic acid, e.g., and may be further coated with a paint or other organic composition. In some cases, paint may be applied directly to the oxide coating before it is sealed.
Because of the difficulties of handling chromium-containing anodizing tank effluents and more recently the stringent restrictions on allowable chromates in the atmosphere, efforts have been directed towards the creation of anodizing methods without chromium. One alternative is anodization in relatively strong aqueous solutions of sulfuric acid.
The problem with this method is that it is difficult to control coating weights and that thin coatings formed by anodizing in sulfuric acid are not as corrosion resistant or paint receptive as like coating weights formed by anodizing in chromic acid. Furthermore, at and above the military minimum aluminum oxide coating weight specification of 3 microns aluminum oxide (600 mg/ft.sup.2) for aluminum or aluminum alloys anodized in sulfuric acid (MIL-A-8625E), the aluminum substrate experiences unacceptable degradation of fatigue resistance.
Thick aluminum oxide coatings (greater than 5 microns) have been applied to substantially pure aluminum and 5000 series alloys by subjecting them to high current density (greater than 13 Amps per square foot) anodization in solutions of sulfuric and boric acids. This method is described in Japanese Patent No. 54-26983 and in the Journal of the Electrochemical Society, Vol. 129, No. 9, pp. 1865-68 (1982).
Efforts to coat modern aircraft alloys of the 2000, 6000 and 7000 series were unsuccessful using the method of these references. In some areas of test panels the coating was too thick and in others, no coating was applied and the metal was discolored. No success was achieved in obtaining uniform, adhesive coatings in the thickness range of about 1 to 3 microns.